Our goal is to shed light on the role of the major histocompatibility complex in the ontogeny and induction of thymus-derived T lymphocytes. By employing irradiated host mice grafted with allogeneic bone marrow cells and thymus rudiments, we will ask whether the polymorphic H-2 antigens determine the efficiency of lymphocyte homing to the thymus and maturation there. Other experiments with mature T cells will ask whether there is a requirement for the recognition of H-2 I region encoded differentiation antigens in the induction of a cytotoxic T cell response. T cell subpopulations will be fractionated using monoclonal antibodies to further investigate the role of T:T interactions in cytotoxic responses. The role and the source of T cell growth factor will also be investigated in our model systems.